In a radio episode of the Hitchhiker's Guide to the Galaxy broadcast in 2004, Ford Prefect lands in a spaceship on Lord's cricket ground in the middle of a test match. He picks up Arthur Dent, who is in the crowd. And when Arthur asks why Ford isn't worried that the people present will see the gleaming spaceship, Ford replies that the crowd can't see the spaceship because it's hidden by a "Somebody else's problem field"; the cricket fans simply don't expect to see a spaceship land in the middle of a match and as a result they can't.

My own experience of this phenomenon was when I was teacher training in Mexico at a small private university in Tlaxcala. The university buildings were on top of a hill and from the second floor you had a magnificent view across the valley of La Malinche, a volcano.

During a coffee break in the afternoon, one of my trainees popped their head round the door and said: "Would you like to see a UFO?" I went outside and together we saw, high above the valley, what appeared to be a small and oscillating grey sphere. Then, after a while, someone noticed as a silver-grey, cigar-shaped object rose up from the horizon. The objects looked a bit like these. Both of them hovered and then drifted off.

"Interesting", some will say. "I wonder if he actually saw a UFO." "Rubbish, they were obviously silver balloons," another will say.

Robert Anton Wilson, the famous free thinker and libertarian, used to say that there are generally two kinds of people in this world: there are those who are capable of going along with all sorts of imponderables like homeopathy and existence of angels. And then there are those people who tend not to believe a single thing, unless it is either common sense or proven.

More interesting than the observation of what might or might not have been UFOs, were the students' subsequent reactions to what had happened. One week later, most of them recalled the experience with embarrassment and non-committal shrugs, though they had no rational explanation for the event. Two weeks later, none of them wanted to mention the incident. Three weeks later, most seemed to have dismissed it from their minds. They had effectively edited it out. Generally speaking, we see what we want to see. We ignore what it suits us to ignore and we seek for confirmation that we are right.

Believers will always find proof for the existence of God: they will find confirmation for his presence in the minutiae of their daily lives, in the prayers that they believe are answered, in the trials that they come through unscathed - in the beauty, meaning and order they perceive in nature. They find their confirmations of the existence of God in friendship and brotherhood, romantic and maternal love. They see it in internally consistent theological arguments in the big bang itself, contradicting the anthropic principle.

But are they wrong to do so? Are the religious sceptics definitely right?

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